Yahoo and Google are teaming up to develop an email system that will feature encryption to keep spies out of the inboxes of users.

The partnership is aiming to develop the system next year, which will be a major step in bringing encrypted communications to a consumer-centric service such as emails.

The encryption will be optional for users, and will have to be activated by users to be utilized.

The project will be based on a kind of PGP encryption, which is a tried-and-tested encryption method that has never been hacked. PGP does not rely on other companies to safeguard the log-in credentials of users, but rather relies on the user holding on to their own encryption key, which will be stored on the user's computer or smartphone.

Alex Stamos, chief information security officer of Yahoo, said that challenges lie ahead for bringing PGP encryption to the public.

One hurdle is that users must first fully understand how PGP works, and that it is not a magical cure for all online security concerns. For example, PGP only adds encryption to the contents of emails, not on the sender or recipient of the email.

"We have to make it to clear to people it is not secret you're emailing your priest," said Stamos at the Black Hat online security conference. "But the content of what you're emailing him is secret."

In addition, utilizing unhackable encryption for email may have legal issues, such as what email provider Lavabit went through last year. The company was forced to shut down after a court demanded Lavabit to give up its encryption keys.

Lavabit was so ordered by the court because it was the old email provider of Edward Snowden, who is famous for revealing that the National Security Agency is spying on emails and other online communications.

Google and Yahoo, with this project that they are going into, will not be going through the same thing as Lavabit if a similar issue arises though, as it is not them that will be possessing the encryption keys.

Back in June, Google made an announcement regarding their plans to develop a spy-proof email. With Yahoo joining the project, it will now involve most of the email accounts in the whole world.

In December of last year, according to ComScore, Google was the leader in email usage with 366 million unique visitors for Gmail, with Yahoo coming in at second with 273 million unique visitors for Yahoo Mail.